Electrical earth borehole logging is well known and various devices and various techniques have been described for such purposes. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, measure electrodes (current sources or sinks) are in employed conjunction with a diffuse return electrode (such as the tool body or an extension thereof). A measured current flows in a circuit that connects a current source to the measure electrodes, through the earth formation to the return electrode and back to the current source in the tool. In the second category, that of inductive measuring tools, on the other hand, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present invention belongs to the former category.
There are several potential modes of operation of a borehole resistivity measurement device. In one, the current at the measuring electrode is maintained constant and a voltage is measured, while in a second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain constant voltage at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. Ohm's law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current.
Numerous examples of the use of focused electrodes for resistivity measurements have been shown in the prior art. One such example is proposed in U.S. Pat. No. 6,348,796 to Evans et al., entitled “Image Focusing Method and Apparatus for Wellbore Resistivity Imaging.” The Evans '796 patent is commonly assigned to the assignee of the present invention and is hereby incorporated by reference herein in its entirety.
In the '796 patent, there is disclosed an apparatus that includes an array of measure electrodes separated from a pad or the body of the instrument by focus electrodes, the pad or body acting as the guard electrode. The focus electrode is maintained at a slightly lower potential than the pad, and the measure electrode is at an intermediate potential thereto. With this arrangement, the current from the measure electrode initially diverges as it enters the formation, then converges (focuses) and then final diverges again to define a depth of investigation. This arrangement tends to reduce the instruments sensitivity to borehole rugosity.
The use of focusing electrodes is also discussed in U.S. Pat. No. 6,600,321 to Evans, entitled “Apparatus and Method for Wellbore Resistivity Determination and Imaging Using Capacitive Coupling.” The Evans '321 patent is commonly assigned to the assignee of the present invention and is hereby incorporated by reference herein in its entirety.
In typical borehole resisitivity measuring instruments, such as disclosed in the aforementioned Evans '321 patent, there are a plurality of resistivity arrays, evenly spaced around the circumference of the tool body (mandrel). Each array comprises a pad surrounding plurality of measurement electrodes and one or more focusing electrodes. There may be, for example, four or six separate resistivity arrays disposed around the circumference of the instrument. The more arrays to be provided, the smaller each array must be in order for all to physically fit around the circumference of the array.
Accordingly, one perceived drawback of such prior art arrangements is that they tend to provide an incomplete image of the borehole, as a result of the gaps that necessarily exist between adjacent resistivity arrays on the instrument. The present invention is intended to address this disadvantage of the prior art.